In the past, a coating type of magnetic recording media has been generally employed. This type of media is generally obtained by coating on a non-magnetic support a magnetic coating composition prepared by dispersing a powdery magnetic material, e.g., a magnetic powder of an oxide such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Berthollide compound composed of .gamma.-Fe.sub.2 O.sub.3 and .gamma.-Fe.sub.3 O.sub.4, CrO.sub.2, etc., a ferromagnetic alloy powder, or so on, into an organic binder. Some conventional binders include vinyl chloride-vinyl acetate copolymer, styrene-butadiene copolymer, epoxy resin, and polyurethane resin. The coated composition is then dried to form a magnetic layer.
In recent years, there has been a demand for recording a large amount of information in a small area of recording material, which is commonly referred to as "high density recording".
With the increased demand for high density recording, there has been an increased demand for the so-called non-binder type of magnetic recording media. Such media contain no organic binders in their magnetic recording layer(s) and have as the magnetic recording layer(s) a thin film(s) of ferromagnetic metal(s) formed by the vapor deposition techniques such as vacuum evaporation, sputtering, or ion plating, or metal plating techniques such as electroplating or electroless plating. Such media have attracted the attention of the art, and various efforts for putting them to practical use have been made.
Conventional coating type magnetic recording media principally utilize metal oxides as magnetic materials. Furthermore, reduction of the thickness of such media is accompanied by a lowering of the signal output. Therefore, reduction of the thickness of the magnetic recording layer, which is necessary for increasing the recording density, is limited. In addition, they must be manufactured by complicated processes using large sized incidental equipment for recovering solvents used in the manufacturing process. Such equipment and procedures may also involve problems of environmental pollution. On the other hand, non-binder type magnetic recording media contain ferromagnetic metals, which have saturation magnetization greater than those of the above described metal oxides. Such media are in the form of a thin film which does not contain any non-magnetic substances such as a binder. Therefore, such media can have very thin magnetic films capable of high density recording. In addition, the manufacturing processes are simple.
Magnetic recording media used for high density recording must use magnetic substances having high coercive force and a reduced thickness. Such being the case, non-binder type magnetic recording media appear to be very promising because it is easy to decrease their thickness one order of magnitude below thickness of the conventional coating type magnetic recording media. Furthermore, such media possess high magnetic flux densities.
In particular, the application of vacuum evaporation techniques to the formation of magnetic recording layers is advantageous because it is not necessary to dispose of waste solutions, unlike metal plating techniques, because of the simple manufacturing process, and because the deposition speed of the magnetic metal film can be increased to a high rate. There are known processes for manufacturing a magnetic film having coercive force and squareness ratio desired for magnetic recording media by utilizing vacuum deposition processes, e.g., an oblique incident evaporation method, as disclosed in U.S. Pat. Nos. 3,342,632 and 3,342,633.
Further, magnetic recording media provided with ferromagnetic metal thin films must have high corrosive strength, abrasive strength, and running stability. The magnetic recording media and a magnetic head are in relative motion at high speed when they contact each other while recording, reproducing, and during erasing operations of magnetic signals. In such operations, smooth and stable running of the magnetic recording media must be ensured. Furthermore, wear or rupture caused by continual contact with a magnetic head must be avoided. It is also desired that there is little or no decrease or unintentional erasure of signals recorded in the magnetic recording medium with the lapse of time, for example, that caused by corrosion or rust during storage.
It has accordingly been proposed that a protective layer be provided in order to improve durability and weather resistance. For example, an organic protective layer is disclosed in U.S. Pat. Nos. 3,466,156, 4,069,360, 4,152,487, 4,152,469 and 4,333,985, German Patent Application (OLS) Nos. 2,929,847 and 3,024,918; a metal protective layer such as Rh is disclosed in U.S. Pat. Nos. 3,516,860 and 4,245,008. In addition, it is disclosed in Japanese Patent Publication No. 4393/70 that a protective layer containing Cr and chromium oxide can be provided by evaporating Cr on a ferromagnetic thin metal film in a vacuum, and it is disclosed in Japanese Patent Publication No. 37615/83 that a protective layer be provided by laminating a Cr layer and a Si-Si oxide layer. However, these protective layers are not completely sufficient because they do not provide sufficient anti-corrosive properties. When the protective layers are used on magnetic recording media, weather resistance after repeated use, for example, in a video tape recorder (VTR) is insufficient. Further, when a ferromagnetic thin metal film is formed on a flexible support by an oblique incident vapor deposition method, the ferromagnetic thin metal film tends to curl with the film on the inside by internal stress, and such curling is not improved even if a protective layer such as a Cr-protective layer is provided on the film.